Numerous techniques exist for the removal of SO.sub.x pollutants from the combustion gases of fossil fuels. These include the conventional wet and dry scrubbers (See for example, U.S. Pat. No. 4,061,476 to Holter et al. and U.S. Pat. No. 4,086,324 to Welty) as well as emerging techniques of furnace sorbent injection (FSI), (See, for example U.S. Pat. Nos. 4,440,100 to Michelfelder et al., 4,655,148 to Winski and 4,960,577 to Torbov et al. backend recycle of FSI products (e.g., U.S. Pat. No. 4,710,305 to Gebhard et al.) or in-duct injection of sulfur reducing sorbents.
In contrast to wet or dry scrubbers these emerging techniques offer significantly lower capital cost and require less space, however their chemical utilization is generally lower than scrubbers and their removal efficiency is typically in the 40 to 80% SO.sub.x range versus 95 to 99% removal for properly designed scrubber systems.
Under the new Clean Air Act, these emerging technologies may provide the opportunity for smaller, older coal-fired boilers that may be near the end of their economic lifetimes to meet the Phase I or Phase II SO.sub.x emission requirements without the capital investment required for wet or dry scrubbers.
One limitation to the broad application of these emerging SO.sub.x control techniques is the increased solids loading on the furnace tubes, backend heat exchanger surfaces, particulate collection devices and ash disposed systems that results from the reaction of sulfur sorbents with SO.sub.x in the flue gas. Typically sorbents will need to be added in a molar ratio of sorbent: sulfur in the range of 2 to 3, to effect a 50 to 70% SO.sub.x capture, translating into a sorbent utilization of only 25 to 40%. The resulting increase in particulate loading from the addition of sorbents and their reaction with gaseous SO.sub.x to form solid sorbent-sulfates can overwhelm the ability of the furnace and other systems to operate. Thus, the application of these SO.sub.x removal techniques has been limited to modest reductions (15 to 40%) or to low sulfur coal (e.g., 0.75% sulfur) applications. In other situations, costly upgrades to boiler particulate collection devices and ash handling may be required thus significantly raising the capital investment required.
In some situations it may be possible to fire lower sulfur coal, either alone or in combination with emerging SO.sub.x control techniques to meet SO.sub.x emission regulations. However, lower sulfur coal typically sells at a premium to moderate or high sulfur coal, is not economically available in all regions, and may impact furnace operation due to variations in heating value, ash and metals contents, and slagging characteristics.